The present invention relates to a method of shaping a granular polytetrafluoroethylene (PTFE) powder.
PTFE, particularly PTFE prepared by suspension polymerization is once pulverized into an average particle size of not more than about 100 xcexcm, and then granulated by various granulation methods such as dry granulation and wet granulation to be a granular powder having an average particle size of from about 100 xcexcm to about 800 xcexcm. The granular powder is used as it is as a molding powder for powder molding. However even if the powder is simply granulated, the granulated powder is poor in powder flowability, and therefore usually after the granulation step, a shaping step is provided for shaping of the granulated powder, thereby enhancing a powder flowability and increasing an apparent density.
For example, a granular PTFE powder obtained by granulating with a ribbon mixer and then with a flash mixer is again treated with a ribbon mixer for shaping at a decreased number of rotations.
In that shaping method with a ribbon mixer, though an apparent density and powder flowability of a granular PTFE powder after the shaping are enhanced, there are problems that granular powders are agglomerated and particles having a large particle size are obtained, which results in shifting to a larger average particle size and increase of a larger particle size region in particle size distribution.
Those problems also arise in case where granular PTFE powders obtained by other granulation methods are subjected to shaping with a ribbon mixer.
An object of the present invention is to provide a method of shaping a granular PTFE powder, which enhances an apparent density and powder flowability without substantially changing an average particle size and a particle size distribution.
The present invention relates to the method of shaping of a granular PTFE powder, characterized in that a granular PTFE powder granulated to an average particle size of from 100 to 800 xcexcm is put in a stirring vessel having two or more rotation axes crossing each other and is subjected to stirring for shaping by rotating the stirring vessel on its rotation axes.
The stirring is carried out under the conditions for increasing an apparent density of the granular PTFE powder without substantially changing its average particle size and particle size distribution.
It is preferable that two or more rotation axes of the stirring vessel are disposed at a right angle to each other. A represented preferable example thereof is a cross rotary mixer.
With respect to the stirring conditions of the cross rotary mixer, it is preferable that a speed of revolution is from 5 to 20 rpm and a speed of autorotation is from 10 to 30 rpm.
A granular PTFE powder to be subjected to shaping may be deagglomerated before the shaping.
As a granular PTFE powder to be subjected to shaping, a granular powder granulated by a dry granulation method or other granulation method can be used.
A granular PTFE powder to be subjected to shaping may contain or does not contain a filler.